Powerline communications (PLC) include systems for communicating data over the same medium (i.e., a wire or conductor) that is also used to transmit electric power to residences, buildings, and other premises. Once deployed, PLC systems may enable a wide array of applications, including, for example, automatic meter reading and load control (i.e., utility-type applications), automotive uses (e.g., charging electric cars), home automation (e.g., controlling appliances, lights, etc.), and/or computer networking (e.g., Internet access), to name only a few.
Current and next generation narrowband PLC are multi-carrier based, such as orthogonal frequency division multiplexing (OFDM)-based (as opposed to single carrier-based) in order to get higher network throughput. OFDM uses multiple orthogonal subcarriers to transmit data over frequency selective channels. A conventional OFDM structure for a data frame includes a preamble, followed by a physical layer (PHY) header, a media access control (MAC) header, followed by a data payload.
PLC channels are known to be highly challenging environments for digital communication because they suffer from periodic bursts of impulse noise, and the channel impulse response also varies over time. As an example, FIG. 1 shows a plot of the channel response of a PLC channel (without any added loads thereon) as a function of time (in msec) for a medium voltage (MV) to low voltage (MV-LV) powerline in the US-grid for the Federal Communications Commission (FCC)-band (150 kHz to 500 kHz). It can be seen that the response of the PLC channel itself is periodically time-varying with a period of about 8 msec, or approximately ½ the AC mains cycle duration. Nulls or troughs in the PLC channel response can be seen to occur every period corresponding to times of minimum channel response.
A conventional preamble structure for a narrowband OFDM PLC standard, e.g. IEEE P1901.2, or G3, includes 8 syncP symbols followed by 1.5 syncM symbols. There is no cyclic prefix between adjacent symbols in the preamble. As known in the art, syncP is a known preamble sequence, and syncM=−syncP. As example, a syncP preamble can be a chirp-like sequence (there many possibilities depending on the chirp rate), a specific binary sequence of 1's and −1's, or a cazac sequence. The definition of the syncP symbol for the FCC band in IEEE P1901.2 involves specifying phases at different tones.
The preamble serves purposes including the following purposes:                1) helps to indicate to other nodes in the PLC network that a transmission is in progress;        2) helps to determine the frame boundary (i.e. the boundary between the preamble and the PHY header, and between the PHY header and the data),        3) can be used to obtain accurate channel estimates, and        4) can be used for frequency offset compensation.        
SyncM symbols help determine the frame boundary. The repetitive syncP symbols also assists in preamble detection as receiver nodes are looking for the repetitive sequence of symbols in the PLC channel to determine whether or not a frame is on the powerline. Multiple syncP's also help in obtaining more accurate channel estimates because averaging the channel estimates across multiple syncP's helps reduce the noise. Improved channel estimates also helps in improving the header decoding performance, especially when the header is coherently modulated with respect to the syncP preamble.